Generating steam from hot process streams is a common method for recovering low value heat particularly in a petroleum refinery. Steam is generated by indirectly heat exchanging the hot process stream with water in a kettle steam generator. A kettle steam generator typically comprises a cylindrical shell and tube heat exchanger with the hot process stream circulating inside the tube bundle and water in the shell. Heat is indirectly transferred from the hot process stream to vaporize the water.
Steam quality is categorized by pressure level. Low pressure steam is typically generated at 241-448 kPa (gauge) (35-65 psig). Medium pressure steam is typically generated at 862-1207 kPa (gauge) (125-175 psig) and high pressure steam is typically generated at or greater than about 4137 kPa (gauge) (600 psig).
The economics of the typical kettle steam generator often do not justify installation, especially as the quantity of recoverable heat is reduced. The expense of kettle steam generators is due in large part to the quantity of instrumentation that is required for each kettle steam generator. Piping, vessels and other auxiliary systems also add cost to the installation. Each steam kettle is typically equipped with a boiler feedwater inlet, a steam outlet, at least two drainage outlets for removing precipitates and at least one steam vent for over pressure relief all with necessary piping and valving. Additionally, instrumentation is required to monitor the water level in the kettle and the steam flow rate through the steam outlet leaving the kettle to regulate with further instrumentation the flow rate of boiler feedwater into the kettle. Installation costs multiply for recovering heat from each additional process stream.
To improve the economics of installing steam kettle generators for heat recovery from multiple process streams it has been proposed to put two discrete heat exchanger tube bundles into a single kettle steam generator thus halving the required instrumentation. Each tube bundle carries a single hot process stream for indirect heat exchange with the water in the kettle. However, the physical installation of this design is cumbersome because tube bundles enter both ends of the cylindrical kettle, thus obstructing access to the interior of the kettle from the traditional location at one end.